Storage systems, such as disk drives, typically include one or more disks or other storage media with a plurality of concentric data tracks. A transducer is typically positioned over a destination track during a write or read operation. Servo head position information is typically recorded on the disk. One common recording format 100 for disk drives, shown in FIG. 1, includes a servo address mark (SAM) pattern 110 that identifies the start of the next set of embedded servo information, as well as a syncMark pattern 120 and a postamble pattern 130 before and after the recorded data 140, respectively.
Typically, a servo demodulator determines where to start searching for a SAM pattern based on the most recently detected SAM. Typically, the servo demodulator searches for the SAM during a time window that is based on the timing of the most recently detected SAM. Once the SAM is detected, the spacing between the SAM and the syncMark is typically known with a high degree of accuracy. Thus, a syncMark location detector can typically search for the syncMark within a relatively small window. Once the syncMark is identified, the syncMark location detector can determine where the data section is located within the recording track. The syncMark detector could miss detecting the syncMark, for example, due to signal defects where the read-back signal on the syncMark is destroyed or distorted beyond the tolerance of the syncMark detector.
A number of techniques have been proposed or suggested for improving syncMark detection. See, for example, U.S. Pat. No. 7,561,649, entitled “Method and Apparatus for Synchronization Mark Detection With DC Compensation,” and United States Published Application No. 2010/0115209, entitled “Method and Apparatus for Detecting a SyncMark in a Hard Disk Drive,” each incorporated by reference herein. One technique computes a metric, such as a Euclidean distance metric, for multiple positions within a syncMark search window and compares the computed metrics to a syncMark metric threshold. If the computed metric for a position satisfies the syncMark metric threshold, the syncMark is declared to be found at that position. If the computed metrics do not satisfy the syncMark metric threshold, then the syncMark metric threshold is increased and the data is physically re-read from the disk for another attempt to detect syncMark using this new syncMark metric threshold. Re-reading the data incurs additional read time and power and also reduces the channel read throughput.
Another technique, shown in FIG. 2, inserts a secondary syncMark 240 (often referred to as syncMark2 or SM2) in the data. When the syncMark detector fails to detect the first syncMark 120, the hard disk drive can rely on the second syncMark 240 to locate and detect the data. The first syncMark 120 and second syncMark 240 are typically separated by a constant length. Typically, a buffer sufficient to store the data between the first and second syncMarks 120, 240 is used to recover the data on the fly for the missing syncMark. The second syncMark 240, however, must be inserted into the data and thus impairs the format efficiency, resulting in a reduced data capacity relative to the technique of FIG. 1.
A need therefore exists for improved techniques for detecting a syncMark in a hard disk drive.